1. Field of the Invention
The present invention relates generally to devices and techniques for installing cable, such as power cable, into coiled tubing such as for use in delivering electrical power to submerged machinery in a petroleum well. More particularly, the invention relates to a novel technique for installing lengths of cable in coiled tubing by at least partially placing the cable or cable shielding under compressive loading at an entry end of the coiled tubing.
2. Description of the Related Art
A number of applications exist wherein power or data signals must be supplied to remote subsea or subterranean locations, such as to drive or control underground equipment. For example, in the field of petroleum extraction and exploitation, it is commonplace to form a wellbore that penetrates a number of subterranean zones, including one or more zones of interest. The zones of interest commonly include geological horizons bearing minerals, such as petroleum, gas, paraffin, and so forth. The minerals flow into the wellbore and are removed by pumping systems either situated at the earth's surface or submerged within the wellbore. In either case, it is often necessary to transmit electrical power or control signals to equipment situated in the wellbore to power or control operation of the downhole equipment.
In one class of equipment used in petroleum and similar wells, a submersible electric motor is coupled to one or more pumps. The electric motor is powered by cables extending from a power supply at the earth's surface. As the electric motor is driven, the pumps displace wellbore fluids, transmitting fluids of interest to a location above the earth's surface for collection and further processing. Certain systems of this type also include ancillary equipment such as fluid separators, injection pumps, and so forth, adding to the complexity and flexibility of the overall system.
In heretofore known devices, several techniques have been employed for transmitting power and control signals to downhole equipment in wells. For example, various types of shielded cable are known and are currently available for transmitting significant levels of electrical power to submerged pumping systems of the type described above. The cables may include multiple conductors, such as for three-phase operation, and are commonly shielded by a flexible, durable metallic casing or armor designed to reduce the risk of damage to the power conductors during deployment and use of the equipment. Control signals may be superimposed on power signals in certain applications, or may be transmitted via separate cables, radio telemetry, or other signal transmission techniques.
In an increasing number of applications, it is useful to dispose cables and signal conductors in extended lengths of conduit extending between the earth's surface and a submerged pumping system, particularly in petroleum and other production wells. One particular type of conduit presently employed in such systems is coiled tubing, which is available in extended lengths capable of being wound around a storage spool. The tubing may be deployed in the well by unwinding it from the storage spool. Lengths of tubing may be spliced together, as required by the depth at which the submerged equipment is deployed. A difficulty arises, however, in insertion of the cable conductors in the tubing or conduit. In many applications it is preferable to pre-install the cable in lengths of conduit to facilitate proper attachment of the cable to the submerged equipment prior to deployment. Presently, power cable is installed in coiled tubing by tensioning the cable during the installation process and drawing the cable through extended lengths of tubing, often several thousand feet long.
Significant disadvantages have been encountered in tensioning techniques for deploying cables in conduit such as coiled tubing. For example, high tension tensile forces are required to draw the cable into the tubing, owing to the weight of the cable and to frictional forces between the interior of the conduit and the exterior of the cable shielding. Such forces can result in damage to the cable during installation process. Moreover, excessive tension in the cable can lead to stretching of the cable during installation. Residual strain resulting from such tension may then be stored along the cable, which remains unrelaxed prior to connection to the powered equipment. During subsequent operation, the residual strain can be relieved, leading to contraction of the cable and, in certain cases, interruption of electrical power between the submerged equipment and wellhead equipment.
There is a need, therefore, for an improved technique for installing conductive cabling, such as power cable into a conduit. In particular, there is a need for a technique for positioning power cable in coiled tubing and the like over considerable lengths that addresses or avoids the drawbacks of prior art techniques, such as damage to the cable or conduit, and storage of residual strain within the cable.